This type of machine usually comprises a bench on which a conveyor belt for moving the slabs to be polished or ground travels in a longitudinal direction. Machines of this type further comprise two bridge support structures arranged on either side of the belt, one at the entry end for the material to be machined and the other one at the exit end for the machined material. The two bridge structures support a spindle-carrying beam at their ends.
The spindle-carrying beam has, mounted thereon, a series of vertical-axis grinding and/or polishing spindles or heads which are arranged in a row and which have, mounted on their bottom end, supports which rotate about the vertical axis of the spindle and on which in turn the abrasive tools are mounted.
The spindle-carrying beam performs an alternating movement in a transverse direction so as to grind the slabs arranged on the conveyor belt over their entire width. The amount of the displacement varies depending on the width of the material being machined.
The tools used are made using hard granular materials such as normally silicon carbide or diamond. In industrial applications abrasive granules usually are not used loose, but agglomerated so as to form an abrasive tool by means of a binding agent (which may be a cement, a resin, a ceramic material or a metal), which has the function of retaining the granules for as long as they perform their abrasive action, before disintegrating and allowing them to fall once worn.
The abrasive tools, as mentioned above, are normally fixed to a support which is rotated by a vertical-axis spindle.
In the case of soft stone materials, such as marble, the tool support, which has a prismatic form with flat surfaces, is generally an abrasive-holder plate.
In the case of hard stone materials, such as granite or quartz, the support is generally a head which imparts a specific movement to the tools which are varyingly shaped and in any case arranged radially. The head may be of the type with oscillating supports (so-called oscillating-segment head) or rotating supports with a substantially horizontal axis for roller-shaped tools (so-called roller head) or rotating supports with a substantially vertical axis for flat tools (called disc head or also planetary or orbital head).
The tools furthermore have a grain size gradually decreasing (from a few hundred micrometres down to a few micrometres) as the slab passes below them. In particular, the first spindle which operates on the slab to be ground has tools with a relatively large grain size, the second spindle has tools with a grain size which is slightly less big and so on, while tools with a very fine abrasive grain are mounted on the last spindle.
The spindle is slidable vertically and imparts to the tools resting on the surface of the material a pressure which may be of a mechanical, hydraulic or pneumatic nature; a pneumatic pressure is by far favoured and in this case the spindle—or “plunger”—is slidable vertically, being operated by a pneumatic pressure.
In this type of machine the spindles and, therefore, the grinding and/or polishing tools pause briefly when there is reversal in the movement since the spindle-carrying beam moves with a rectilinear alternating motion transversely with respect to the direction of feeding of the material.
This brief pause results in a very slight localized depression in the material which is sufficient, however, to create visible shadow zones, in particular on the ground or polished surface of particularly delicate dark materials.
In an attempt to the solve this problem, different machines have been devised, including that described in international patent application WO2011064706, which envisages a spindle-carrying beam and spindle-carrying structures rotating about a vertical axis on which the spindles are mounted in an eccentric position. In this type of machine in which the head is defined as being orbital, the relative movement of tool and slab is a combination of movements consisting of:                the alternating movement of the beam in the transverse direction;        the movement of the material underneath the beam;        the rotation of the grinding/polishing head/plate mounted on the spindle;        the revolving movement of the spindles about the axis of rotation of the spindle-carrying structure;        
There exists moreover another type of machine in which a plurality of bridge structures, arranged transversely with respect to the bench, are provided. One or two grinding and/or polishing spindles displaceable along the bridge structure are mounted on each bridge. In the case where there are two spindles per bridge structure, each spindle is movable independently in the transverse direction, namely each spindle is provided with its own drive, so that it may be moved independently along the bridge structure. Moreover, the bridge structures perform an orbital movement, being suspended on four link-rods, so that the amplitude of the orbital movement is a few centimetres, equal to the length of the link-rods.
In this type of machine, each tool is moved with a motion composed of:                a rotational motion about the vertical axis of the spindle;        an alternating transverse displacement due to the movement of the spindle along the bridge;        an orbital motion due to the movement of the bridge on the suspension rods;        a continuous longitudinal displacement due to the feeding of the material on the bench.        
The machines described above, while being widely used, are not without drawbacks.
In fact, although the trajectories of the machine tools described above are sufficient to limit or avoid the aforementioned problems, said tools have an extremely complex design. In fact, in the first case, a structure for eccentrically supporting the spindles is provided, said technical solution complicating significantly the spindle movement mechanisms. In the second case, in an attempt to achieve uniformity in the surface machining of the slabs, each spindle is provided with a drive and has an independent movement, and therefore the system becomes very costly and complex.